Eosinophils, immunoglobilin (Ig)E and cytokines have important roles in defence mechanisms against helminths. In this study, the influence of HTLV-1 infection, characterized by a Th1 type of immune response, was evaluated on the cytokine pattern and parasitic specific IgE response in patients with strongyloidiasis. Patients were divided into four groups: strongyloidiasis without HTLV-1 infection, strongyloidiasis with HTLV-1, HTLV-1 without strongyloidiasis and controls without either helminth infection or HTLV-1. The cytokine profile was determined in supernatants of mononuclear cells stimulated with Strongyloides stercoralis crude antigen and the parasite specific IgE was measured by ELISA. Patients coinfected with HTLV-1 had higher levels of interfron (IFN)-g and interleukin (IL)-10 (P , 0´05) and lower levels of IL-5 and IgE (P , 0´05) than patients with strongyloidiasis without HTLV-1. There was an inverse relationship between IFN-g and IL-5 (P 0´01; rs 2 0´37) and between IFN-g and parasite specific IgE (P 0´01; rs 2 0´39), and a direct relationship between IFN-g and IL-10 (P 0´04; rs 0´35). These data show that coinfection with HTLV-1 decreases IL-5 and IgE responses in patients with strongyloidiasis consistent with a relative switch from Th2 to Th1 response. Immunological responses such as these are important in the control of this helminthic infection.